Electro-chemical sensor for the detection of reducing gases, in particular carbon monoxide, hydrazine and hydrogen in air

ABSTRACT

A sensor consists of a catalytically active measuring electrode communicating with a non-polarizable atmospheric oxygen electrode by way of an aqueous acid electrolyte. The electrolyte used is an aqueous gelatinous polymeric adhesive in which ionogenic substances are dissolved. Due to the adhesive properties of the electrolyte, a 3-phase boundary of air-catalyst-electrolyte is formed at the transition to the measuring electrode and substantially determines the measuring process.

BACKGROUND OF THE INVENTION

This invention relates to an electro-chemical sensor for measuringcarbon monoxide, hydrogen or hydrazine in the surrounding atmosphere,comprising a polarizable, catalytically active measuring electrode whichis exposed to the surrounding air and communicates by way of an aqueousacid electrolyte with a non-polarizable atmospheric oxygen electrode ascounter electrode.

In immission monitoring, and to some extent also in emission monitoring,it is necessary nowadays to detect traces of gas at extremely lowconcentrations. This requires measuring processes of the utmostsensitivity. For example, limiting exposure to carbon monoxide is one ofthe most extensive and urgent problems in many fields or industry.Environmental pollution by carbon monoxide is caused by incompletecombustion, e.g. of engine fuels or domestic fuels. Due to the toxicityand risk of explosion, combined with the impossibility of recongnizingthe pollutant by color, odor or taste, monitoring of the concentrationsis necessary and this should be as far as possible continuous.

It was therefore an object of the present invention to develop a gasdetector to measure the concentrations of harmful substances. The deviceshould be suitable for use by technically unskilled operators andtherefore relatively easy to handle, should function according to asimple, reliable method of measuring and should be ready for takingmeasurements as soon as it is switched on.

Apart from purely chemical and physical measuring methods,electro-chemical methods are frequently used for the determination ofcarbon monoxide, hydrogen and hydrazine. Measuring cells with3-electrode arrangements have been proposed for the measurement ofcarbon monoxide (see. H. W. Bay, K. F. Blurton, H. C. Lieb, H. G. Oswin,International Laboratory, vol. 1, 1972, No. 5, pages 37 to 41; U.S. Pat.Nos. 3,824,167 (1974) 4,013,522 (1977); German Offenlegungsschrift No.2,155,935 (1972). These are analogous to the usual half cellarrangements used in electro-chemistry, in which carbon monoxide isoxidized in aqueous solutions at noble metal electrodes, mainlyplatinum, at potentials of +0.9 to 1.5 V against a standard hydrogenelectrode, in accordance with the following equation:

    CO+H.sub.2 →CO.sub.2 +2H.sup.+ +2C

The measuring cell consists of measuring electrode, a counter electrodeand a reference electrode. The potential of the measuring electrode withrespect to the reference electrode is adjusted by means of an electronicpotentiostat (voltage source) and maintained constant within therequired range. The carbon monoxide diffusing towards the measuringelectrode is oxidized at that electrode and an electric currentproportional to the carbon monoxide concentration is produced.

Alternative proposals include measuring cells consisting of 2-electrodearrangements where carbon monoxide is oxidized at the anode and thecounter electrode (cathode) either contains an active substance (oxidesor mixed oxides of transition elements) whose redox potential polarizesthe anode to the desired electrode potential of +0.9 to 1.5 V (seeGerman Offenlegungsschrift No. 23 16 365) or consists of a gas electrodeat which atmospheric oxygen, for example, is reduced. The dimensions ofthe cathode are selected so that the potential of the cathode does notvary with the measuring currents produced.

Arrangements comprising a reference electrode or a counter electrodefunctioning as a reference point do not operate completelysatisfactorily because the reference potential varies over a period oftime. This means that the potential of the measuring electrode alsochanges so that the primary current is liable to increase in thepositive or negative direction due to the reduction at atmosphericoxygen or the formation of oxide layers on the surface of the measuringelectrode. Since it is virtually impossible to achieve a constantpotential when a reference electrode is used, this measuring system ismore susceptible to trouble.

In sensors which are used in portable gas warning instruments or insmall telemetering equipment, as in the described invention, it isadvisable to replace the liquid electrolytes by solid electrolytes or tofix the electrolytes in order to avoid outflow of the acid frequentlyused as electrolyte. One known method is immobilization simply bysuction, e.g. in asbestos powder, quartz powder or a porous organic orinorganic matrix, e.g. porous PVC filter discs or porous glass filterpaper. Fine grained ionic exchange resins which absorb aqueouselectrolytes may also be used to fix the electrolyte. When such sensorcells are in operation, the volume of the electrolyte is reduced due toloss of water by evaporation, with the result that the contact betweenelectrode and electrolyte can no longer be maintained and thesensitivity of measurement is therefore reduced.

SUMMARY OF THE INVENTION

It is an object of the present invention to develop electro-chemicalsensors for the measurements of carbon-monoxide, hydrazine or hydrogenin air on the basis of immobilized electrolytes with rapid response anda very low primary current.

To solve this problem in accordance with the invention, the electrolyteconsists of an aqueous, gelatinuous polymeric adhesive in whichionogenic substances are dissolved, and the measuring electrode is acurrent-abducting matrix containing the catalyst, the adhesiveproperties of the electrolyte together with the electrode resulting in a3-phase boundary of ambient air- catalyst-electrolyte.

The acid aqueous phase of the electrolyte is a homogeneous system, i.e.the aqueous acid constituents are incorporated by polymerization whencross-linking takes place. The current producing electrochemicalreaction takes place on those catalyst particles which have part oftheir surface embedded in the electrolyte but are otheriwse exposed toambient air and are in addition in electrical contact with the matrix.The essential criteria for the measuring effect is thus the abovementioned 3-phase boundary.

By "matrix" is meant here a porous sheet-like structure, e.g. a fleeceor grid of electrically conductive materials.

The good adhesive properties of the electrolyte surface ensures goodmechanical adherence of the two electrodes. The intimate contact betweenelectrodes and electrolyte ensures optimal development of the 3-phaseboundary which is so important for the electrochemical reaction. Inconventional gas diffusion electrodes having a 3-phase zone ofelectrode-electrolyte-gas, the gas must either be delivered to theelectrode under pressure or the electrode must be rendered hydrophobicin order to prevent over filling of the electrodes system of pores withelectrolyte and spilling of the electrolyte, (see W. Vielstich,Brennstoffelemente, pages 4 and 5, publishers Chemie, Weinheim 1965).

BRIEF DESCRIPTION OF THE DRAWING

The basic construction of the measuring cell is shown in the FIGURE andis described below with reference by way of example to a CO-cell.

DETAILED DESCRIPTION OF THE INVENTION

A cylindrical body 1 of plastic, e.g. polypropylene, contains an aqueousgel electrolyte 3. A measuring electrode 2 and counter electrode 4 arenot constructed as gas diffusion electrodes as in fuel cells, but arefirmly bonded to the gel electrolyte due to the adhesive properties ofthe latter. Both the measuring electrode and the counter electrode maybe differently constructed according to the given measuring problem andmay contain different catalysts. They will be described in more detaillater.

A component to be measured (CO) passes through a diffusion barrier 5, inthis case a measuring orifice, to reach the measuring electrode 2 whereit is oxidized to CO₂. At the same time, the oxygen in an air chamber 6is reduced to water at the counter electrode. The dimensions of the airchamber are such that the atmospheric oxygen used up at the electrodecauses only a slight change in the oxygen partial pressure of the air.Since the currents produced are in the range of nano amperes, they areamplified by means of a current amplifying circuit. For this purpose,the measuring electrode and counter electrode are connected to asuitable measuring amplifier 7 and resistor 8, the output of which ismeasured by meter 9.

The following examples for the measurement of CO, H₂ and hydrazine aregiven to enable a clear understanding of the sensors produced accordingto the invention. These cells are, of course, also suitable for thedetection of those reducing substances which occur as crosssensitivities in the following examples. The examples describe themanufacture of the individual electrodes for the electro-chemicaloxidation of CO, H₂ and hydrazine and the corresponding counterelectrodes. They also describe the preparation of the electrolytesaccording to the invention. The examples show that using the cellaccording to the invention results in a gas measuring cell which is ofthe highest sensitivity as well as being sturdy and requiring littleservicing.

EXAMPLE 1

A mixture of 15 g of arylic acid amide, 12.5 g of urea, 7.5 g ofacetamide, 2.5 g of propionic acid nitrile and 50 g of a 30% phosphoricacid is heated to 60° C. in a glass beaker, and solution polymerizationis initiated by the addition of 2.5 ml of a saturated potassiumperoxidisulphate solution. After the onset of polymerization, themixture, which is now in the form of a thick liquid, is poured into acylinder 2 cm in height and of 1.6 cm internal diameter. Aftertermination of polymerization and cooling, the electrolyte sets to asolid, sticky gel.

2 discs 1.6 cm in diameter are punched out of a sheet of carbon fleececa. 0.2 mm in thickness, and each disc is brought into contact with athin platinum wire. These discs, which serve to conduct away thecurrent, are placed on the two sides of the electrolyte surface, wherethey become firmly anchored by the good adhensive properties of the gel.Finely divided platinum black is then advantageously applied to theelectrolyte with carbon fleece in much the same way as in a screenprinting process by applying ca. 5 mg of platinum per cm² through a finemeshed plastics net.

The resulting measuring cell is provided with a diffusion barrier in theform of an orifice 5×0.3 mm in diameter on the side of the measuringelectrode and is sealed off with a plastic cap on the side of thecounter electrode (cathode). The volume of the air chamber above thecathode is ca. 1 cc. The measuring sensitivity for CO is of the order of8 nA/ppm.

EXAMPLE 2

Measuring cells according to Example 1 were produced in which 50 g of Mg(C10₄)₂ were added as a hygroscopic substance in addition to the polymercomposition described. The measuring cells manifested the same behavioras in Example 1 but only slight strinkage of the gel due to water losswas observed, even after 3 months.

EXAMPLE 3

In the manufacture of this cell, the gel forming substance was firstseparately prepared by solvent polymerization, i.e. a mixture of 15 g ofacrylic acid amide, 12.5 g of urea, 7.5 g of acetamide and 2.5 ml ofpropionic acid nitrile in 200 ml of water was reacted at 60° by theaddition of 2.5 ml of a saturated K₂ S₂ O₈ solution. The reactionproduct was precipitated with acetone, filtered and dried. Theelectrolyte solution consisting of 50 ml of 2 N sulphuric acid and 50 gof Mg (C10₄)₂ was added to 20 g of this reaction product, which was thenpoured into the cell body. When gel formation was completed, theelectrolyte was covered on both surfaces with carbon fleece discs 0.2 mmin thickness which had previously been brought into contact with a thinplatinum wire. 5 mg of platinum black per cm² was then applied to bothcarbon fleece as in Example 1. The resulting measuring cell was providedwith a diffusion barrier and plastics cap as in Example 1. Thesensitivity of measurement for CO was ca. 8 nA/ppm.

EXAMPLE 4

A measuring cell was produced, using carbon fleece to conduct awaycurrent and platinum black as catalyst. To ensure that the platinumblack would be bonded to the fleece, the fleece was first soaked in adilute polymer solution of 10 parts of water and 1 part ofpolyacrylamide and was then uniformly coated with platinum black (5 mgof platinum per cm²). The electrodes treated as described above, weredried in air and then glued to both sides of the gel electrolyte. Theaqueous gel electrolyte consisted of 2 N sulphuric acid and the reactionproduct of polyacrylamide, urea and acetamide and was prepared as inExample 1.

The resulting measuring cell, which had a diameter of 1.6 cm and aheight of 2 cm, was provided with a diffusion barrier (restrictororifice) as in Example 1 and sealed off with a plastic cap on the sideof the cathode. The cell had a measurement sensitivity for carbonmonoxide of 12 nA/ppm and a delay time t₉₀ of ca 2 minutes. Themeasurement sensitivity for hydrogen was found to be 20 nA/ppm and thedelay time t₉₀ ca. 90 seconds.

EXAMPLE 5

For the production of this measuring cell, a gel electrolyte based onpolyacrylamide and phosphoric acid was prepared. The gel electrolyte wasobtained by the polymerization at 70° C. of acrylic acid amide (10% byweight) in a 60% by weight phosphoric acid solution with the addition of1% by weight of K₂ S₂ O₈ solution. To produce the electrodes, carbonfleece was used to conduct away current and platinum black as acatalyst. To bond the platinum black to the fleece, the fleece was firstsoaked in a dilute polymer solution consisting of 20 parts of water and1 part of polyacrylamide and then uniformly coated with platinum black(5 mg of Pt/cm²). After repeated impregnation with the dilute gelelectrolyte solution to fix the platinum black powder, the resultingelectrode was dried in a desiccator. Two discs 1.6 cm in diameter werethen punched out and each was brought into contact with a thin platinumwire. The resulting electrodes were placed on the electrolyte surfaceson both sides and firmly anchored to the gel by light application ofpressure.

The sensitivity of measurement of the measuring cell for CO was 12nA/ppm and its delay time t₉₀ ca. 2 minutes.

EXAMPLE 6

Traces of hydrazine gas were measured with a measuring sensor similar tothose described in the above Examples. The electrolyte was prepared bythe polymerization of 15 g of acrylic acid amide, 12.5 g of urea and 1.5ml of formaldehyde solution (35% by weight) in a solution of 60 g of Mg(C10₄)₂ in 50 ml of water. A carbon fleece to which a thin gold layerhad been applied by vaporization under vacuum was used as working ormeasuring electrode.

The counter electrode was an atmospheric oxygen electrode with platinumas catalyst, the preparation of which is described in Example 1 to 6.The specific sensitivity for hydrazine was ca. 1,000 nA/ppm.

We claim:
 1. In an electro-chemical sensor for the measurement of carbonmonoxide, hydrogen or hydrazine in ambient air, comprising a catalystcontaining measuring electrode exposed to ambient air and communicatingthrough an aqueous acid electrolyte with a non-polarizable counterelectrode, the improvement comprising:(a) a body of an aqueoushygroscopic polymer gel in which ionogenic substances are dissolved, thegel body having sticky surfaces, (b) the measuring electrode comprisinga porous sheet-like matrix, coated with catalyst particles, (c) thematrix being firmly anchored at the surface of the polymer gel in aself-supporting manner and the catalyst particles being embedded withpart of their surfaces in the gel and with the remaining surfaceportions protruding from the gel.
 2. The sensor according to claim 1,wherein the matrix consists of a meshed electrically conductive grid,which is applied to the gel electrolyte by pressure.
 3. The sensoraccording to claim 1 or 2, for the measurement of carbon monoxide andhydrogen, wherein the matrix and the counter electrode consists of agraphite fleece and the catalyst is platinum.
 4. The sensor according toclaim 1 or 2, for the measurement of hydrazine, wherein the matrixconsists of a carbon fleece coated with gold and the counter electrodeconsists of a carbon fleece coated with platinum black.